The principle of generating electrical power using refuse materials as the primary fuel has become a very attractive operation particularly for municipally owned power generating systems. Such refuse burning power plants offer a solution to the increasingly troublesome burden of trash disposal further motivated by the economics of utilizing the considerable fuel value of the refuse collected.
However, a particularly troublesome and costly problem encountered in the boilers used in such plants is the excessive breakage of the tuyere structures. The tuyere acts as a barrier to combustible materials falling between the rear boiler wall and the moving stoker grates carrying the refuse fuel through the boiler. Also, these plates function to direct the flow of incoming air through the grate in a desirable manner for combustion and cooling purposes.
The prior conventional boiler constructions utilize a tuyere assembly commonly referred to in the boiler industry as an "air seal", wherein a cast metal plate is mounted to a rear tuyere support wall. the prior tuyere plate included a fish-hook shaped end which is disposed within a trough or recess provided in the support wall. The plates were wedged in the recess by a metal shim. The forward portion of the plate extended over a portion of the path of travel of the grate bars comprising the stoker grate. This construction, while providing relatively easy installation and replacement of a tuyere plate, also provided a tuyere plate essentially fixed relative to engagement with any debris movably carried on the stoker grates.
The nature of such refuse burning boiler operations requires minimum maintenance and downtime schedules in order to approach the necessary economics which justify such operations.
While wear and breakage of the conventional boiler tuyere assembly are expected to occur, the rate of failure of these prior conventional tuyere assemblies in refuse burning power plants has exceeded all prior expectations. In one plant operation, the average life of a conventional tuyere plate was less than three weeks. Scheduling an offline downtime once a month for maintenance and replacement parts failed to reduce the loss of efficiency during this interval caused by operating the boiler with a less than effective tuyere air seal. Studies have shown that inadequate tuyere function may reduce boiler efficiency by as high as approximately 5 percent. This represents a very significant cost on top of the increased losses due to downtime of the boiler and the labor and materials for replacement parts. These losses on an annual basis may exceed seventy-five thousand dollars per boiler.
Such costs dramatically reduce the economic efficiency of the refuse burning power plant concept. Prior to the present invention, this problem has been unsolved and has had detrimental effect of the planning of other communities to build such systems in spite of the relative urgency of solving trash disposal problems and the beneficial effects of fossil fuel conservation. It has been found that the major breakage of the prior tuyere plates and/or support occurs due to pieces of metal or glass carried in the refuse. Upon melting in the high temperature they tend to adhere to the surface of the stoker grate to form a relatively large build-up of knobby-like obstructions. As the moving grate passes under the forward end of these plates, these obstructions engage the leading end of the plate causing significant stresses to the plate and to the tuyere support structure. It has been found that such stresses are often sufficient to break either the plate, the support upon which it is mounted, or both. The breakage caused by such obstructions lead to the significant problems noted above.